Gettler says that although other studies have compared testosterone levels in men of various relationship and parenting status, this is the first study to monitor hormonal changes during the transition from one stage to another.

Before child (BC)

At the beginning of the study period the single men were, on average, approximately 21 years old and only 16 per cent were married with children. At the time of follow up, four and a half years later, 50 per cent were married with children.

"We found single men with higher testosterone levels were more likely to become partnered fathers, which suggests that testosterone helps boost men's ability to attract and secure a mate," says Gettler.

At both stages, the participants were asked to collect two saliva samples: the first immediately after waking, the second immediately before bed. Gettler and colleagues also collected socioeconomic, demographic, physical and behavioural data such as body mass index (BMI), self-reported stress levels and quality of sleep.

After delivery (AD)

The team found men that had become fathers had significantly reduced levels of testosterone.

"We found that the median decline in testosterone levels was minus 26 per cent in the morning and minus 34 per cent at night," says Gettler.

Moreover, the researchers found that the largest decreases were in fathers with newborns aged less than one month.

They then evaluated whether men who contributed to daily childcare differed from those who were 'hands-off' dads.

"Men who were the most involved, those who cared for their children for three plus hours per day, their evening testosterone was on average 80 per cent of the level of those men who weren't involved," says Gettler.

"It's a case of behaviour driving and/or reinforcing biology."

He says humans produce children that are completely dependent on caregivers for a decade or more and that raising them is necessarily a cooperative endeavour.

"Mothers would have needed help raising multiple children at once, and fathers were in a good position to assist them during human evolution, as they are now."

But, what isn't yet known is what happens over time and whether a father's testosterone levels return to normal when children reach a certain age.

Dr Carolyn Allan, an endocrinologist at Prince Henry's Institute in Melbourne and advisor to Andrology Australia, says the paper sets the scene for studying the cause and effect nature of behavioural characteristics factors likely to influence men's testosterone production.

"There are interesting questions for basic science and the link between psychological factors and the neuroendocrine system &hellip; if we could find the mechanism it would give rise to more detailed understanding of testosterone production," she says.

But she adds that there are some questions around the measurement of testosterone in saliva.

"The most robust way is to use a blood sample and mass spectrometry. Also, testosterone levels have a significant degree of variability. The recommendation here [in Australia] is that you would have at least two morning levels."